The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. A common solution is to protect the surfaces of such components with an environmental coating system, such as an aluminide coating or a thermal barrier coating system. The latter includes an environmentally-resistant bond coat and a layer of thermal barrier coating (TBC) of ceramic applied over the bond coat. Bond coats are typically formed of an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel, or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. During high temperature excursions, these bond coats form an oxide layer or scale that chemically bonds the ceramic layer to the bond coat.
Zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides has been employed as the material for the ceramic layer. Yttria-stabilized zirconia (YSZ) is widely used as the ceramic layer for TBCs because it exhibits desirable thermal cycle fatigue properties. In addition, YSZ can be readily deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). Notably, YSZ deposited by EBPVD is characterized by a strain-tolerant columnar grain structure, enabling the substrate to expand and contract without causing damaging stresses that lead to spallation.
As is known in the art, stabilization inhibits zirconia from undergoing a phase transformation (tetragonal to monoclinic) at about 1000.degree. C. that would otherwise result in a detrimental volume expansion. Shown in FIG. 3 is the phase diagram of the zirconia-rich region of the zirconia-yttria system. The phase diagram shows that, at room temperature, a more stable tetragonal phase is obtained and the undesirable monoclinic phase is avoided if zirconia is stabilized by at least about six weight percent yttria. The phase diagram further shows that an yttria content of seventeen weight percent or more ensures a fully stable cubic phase. Testing reported by S. Stecura, "Effects of Compositional Changes on the Performance of a Thermal Barrier Coating System," NASA Technical Memorandum 78976 (1976), showed that plasma sprayed YSZ coatings containing six to eight weight percent yttria were more adherent and resistant to high temperature cycling than YSZ coatings containing greater and lesser amounts of yttria. Conventional practice in the art has been to stabilize zirconia with at least six weight percent yttria, and more typically about six to eight weight percent yttria (6-8% YSZ). Exceptions to this trend have generally been limited to plasma sprayed zirconia that is stabled by a combination of oxides, such as yttria with substantial levels of other oxides as taught in U.S. Pat. Nos. 4,132,916 and 4,996,117.
Though thermal barrier coatings employing zirconia stabilized by 6-8% yttria perform well in gas turbine engine applications, and significant advances have been made with bond coat materials and coating processes that further promote the adhesion, environmental-resistance and durability of YSZ coating systems, still greater improvements are desired for more demanding applications.